To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, the self-aligned double patterning (SADP) process of adding film to opposite sidewalls of lines of a resist pattern resulting from ArF lithography for thereby forming two patterns with half line width from one pattern is successful in manufacturing microelectronic devices at the 20-nm node in a mass scale. As the miniaturization technology for microelectronic devices of the next generation 10-nm node, the self-aligned quadruple patterning (SAQP) which is double repetition of SADP is a candidate. It is pointed out that this process is quite expensive because formation of sidewall film by CVD and processing by dry etching are repeated several times. Extreme ultraviolet (EUV) lithography of wavelength 13.5 nm is capable of forming a pattern with a size of the order of 10 nm via single exposure, but suffers from the problems of still low laser power and low productivity. As the miniaturization technology comes to the deadlock, the development of three-dimensional devices such as vertically stacked flash memories typically BiCS is started, but expected to be a high cost process.
Recently, a highlight is put on the organic solvent development again. A positive resist composition featuring a high resolution is subjected to organic solvent development to form a negative pattern. As the ArF resist composition for negative tone development with organic solvent, positive ArF resist compositions of the prior art design may be used. Such a pattern forming process is described in Patent Document 1.
In the process of forming a negative tone pattern via organic solvent development, a film from which a robust protective group such as cyclic structure having dry etch resistance has been eliminated is left as the negative pattern. Thus the film is short of dry etch resistance. This serious problem must be overcome before the negative pattern formation via organic solvent development can be implemented.
On the other hand, studies have also been made on the negative pattern formation via development in alkaline aqueous solution. Known resist compositions used in this process include a negative resist composition of polarity switch type comprising a base resin comprising recurring units having γ-hydroxycarboxylic acid which forms lactone ring by PEB (see Patent Document 2), a negative resist composition comprising a copolymer comprising alcoholic hydroxyl-containing (meth)acrylate units and fluoroalcohol-containing units and a crosslinker (see Patent Document 3), and negative resist compositions of crosslinking type comprising a crosslinker and a combination of α-hydroxyacrylate and lactone units (see Patent Document 4), α-hydroxyacrylate and fluoroalcohol units (see Patent Documents 5 to 7), and mono(meth)acryloyloxypinacol and fluoroalcohol units (see Patent Document 8).
Of these, Patent Document 2 describes a negative resist composition of polarity switch type, not resorting to crosslinking reaction, in which γ-hydroxycarboxylic acid units incur swell of the pattern after development. Patent Documents 3 to 7 relate to negative resist compositions of crosslinking type. Although the negative pattern formation by cooperation of alcoholic hydroxyl group and crosslinker has the problems of bridging between pattern features and pattern collapse due to swell, it is observed that the incorporation of fluoroalcohol units has a swell-reducing effect. Moreover, as recent examples of negative pattern formation by polarity switch, there are proposed base resins having polar units such as tertiary hydroxyl group, tertiary ether bond, tertiary ester bond or acetal bond as the polarity switch group. Of these, a polymer using a polar unit having one tertiary hydroxyl group is unlikely to swell after development. However, the difference of dissolution rate in developer between unexposed and exposed regions is insufficient, which raises the problem that a footing occurs at the bottom of a line-and-space pattern, that is, pattern features take a tapered shape. See Patent Documents 9 and 10 and Non-Patent Document 1.
All the negative pattern forming processes mentioned above are effective to some extent in forming pattern features with a size of the order of 100 nm. However, their performance is insufficient in forming pattern features with a size of finer than 100 nm, because pattern bridging and collapse due to swell, and footing at the pattern bottom inevitably occur. Although active efforts have recently been devoted on the negative pattern forming process via organic solvent development, the organic solvent used as the developer is more expensive than conventional alkaline developers. From the standpoint of etch resistance improvement, it is desired to have a negative resist composition which is amenable to conventional alkaline development at a high resolution and allows a robust backbone structure to be left in the film after development.